In magnetic resonance methods for examining the body of a live human patient for medical purposes, e.g. to obtain image and/or spectroscopic data, the body under investigation is placed in a static magnetic field to define an equilibrium axis of magnetic alignment in the region of the body being examined. A radio frequency (r.f.) magnetic field is then applied to the region being examined in a direction orthogonal to the static magnetic field direction to excite magnetic resonance in material, typically in hydrogen protons, in the region, and the resulting r.f. signals are detected and analysed. During this sequence of operations one or more gradients are normally imposed on the static magnetic field to cause excitation of magnetic resonance preferentially in the region of the body to be examined, to encode spatially the detected r.f. signals, and also for other purposes such as flow encoding.
When microscopic examination of a very small internal region of a body is required, to improve signal-to-noise ratio detection of the r.f. signal is carried out by means of one or more electric coils carried on a probe inserted into the body so as to lie adjacent the region of interest. To obtain high, e.g. 1 micron, resolution in the region of interest, relatively steep magnetic gradients must be applied across the region of interest for spatial encoding purposes etc. In addition, since water molecules in most human tissues have a relatively high diffusion coefficient such that water molecules in the tissue will typically move a distance of about 8 microns in 10 milliseconds, collection of data must be effected rapidly making even steeper magnetic field gradients desirable. The application of sufficiently steep magnetic fields by the kinds of coil arrangements conventionally used in magnetic resonance apparatus and methods has proved very difficult.